1. Field of the Invention
The present invention generally relates to an SSB modulator used in the microwave region.
2. Description of the Prior Art
A variety of identification methods for detecting and identifying a moving object have been developed in recent years since such identification systems were put into practical use. The system referred to above is used to detect whether or not a moving object such as an automobile vehicle or freight car is present in a certain supervisory zone. It also identifies the object if such a moving object is observed. According to the identification system, when a microwave which is not modulated (the frequency is, for example, 2.45 GHz) is radiated from an interrogator to a transponder, it is received and modulated by the transponder on the basis of information stored in the transponder. Then, the modulated microwave is radiated again to the interrogator which in turn demodulates the modulated microwave, thereby obtaining information about the moving object. Thus, the presence or absence of the moving object, and the kind of the moving object, are detected in the identification system. Since the wave sent and received by the interrogator in the above-described identification system has the same frequency, the circuit construction can be simplified if the transponder employs an SSB modulation system and the interrogator employs the homodyne wave-detection method.
Conventional methods for SSB modulation are illustrated in FIGS. 6 and 7. In the method of FIG. 6, the Selective-Filtering Method, the carrier wave and the modulation wave are multiplied in a balanced modulator 11. Then, only the necessary side band is extracted by a side band filter 12 from outputs of the balanced modulator 11. A high-pass filter or a low-pass filter is used for the side band filter 12, which is required to have a considerably sharp cut-off characteristic. On the other hand, the method shown in FIG. 7 is called a Phase-Shift Method, according to which the modulation wave and the carrier wave are respectively divided into two. One each of the modulation wave and the carrier wave, after division, are multiplied in a balanced modulator 15, while the others of the modulation wave and the carrier wave are respectively delayed 90.degree. in phase by phase shifters 16 and 17, and then mixed in a balanced modulator 18. Thereafter, output signals from the balanced modulators 15 and 18 are added in an adder 19, to obtain an SSB signal.
However, according to the above-described SSB modulator employing the Selective-Filtering Method, since only the necessary side band is extracted from the resulted modulated wave by the side band filter 12, the SSB modulator is hard to use in the microwave region because the side band frequency is only 20K-200 KHz away from the carrier wave frequency of 2.45 GHz. In other words, since the USB (upper side band) is close to LSB (lower side band), the side band filter 12 should have a steep cut-off characteristic. However, the microwave band filter is formed in many cases in the pattern of a copper foil on a printed board, for which the board itself has a low Q (quality factor) of 100, and accordingly, the above-described side band filtering with filter 12 is difficult to achieve.
Meanwhile, according to the SSB modulator based on the Phase-Shift Method, every frequency in the modulated wave should be shifted 90.degree. phase difference by the phase shifter 17. However, such a phase shifter is difficult to realize, and therefore, the SSB modulator employing the Phase-Shift Method can only be used approximately in a limited frequency region. Moreover, since the circuit construction of the SSB modulator using the Phase-Shift Method is complicated, the above SSB modulator is hard to employ as a modulator for detection of a moving object which requires it to be compact in size and light in weight.